skbuff.h: fix kernel-doc warning for headers_end

Fix kernel-doc warning in <linux/skbuff.h> by making both headers_start
and headers_end private fields.

Warning(..//include/linux/skbuff.h:654): No description found for parameter 'headers_end[0]'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>